The immune system defends an organism against pathogen infection, cellular transformation, and physical or chemical damage. When the immune system is less active than normal, immunodeficiency or immunosuppression occurs, resulting in life-threatening infections or cancer. Immunosuppression can either be the result of a disease, or be produced by pharmaceuticals or an infection. Systemic immunosuppression has been found to be associated with abnormal myelopoiesis secondary to tumor growth, myelosuppressive therapy, and growth factor administration and subsequent expansion/mobilization of bone marrow derived immunosuppressive cells. These myeloid-derived suppressor cells (MDSCs) reduce activated T-cell number and inhibit T-cell function by multiple mechanisms, thereby leading to immunosuppression and tolerance. Thus, MDSCs have a pro-tumor role. In addition, MDSCs have pleiotropic activities that include induction of mutations in the tumor microenvironment, promotion of angiogenesis and metastasis, and direct support of both neoplastic growth and inflammatory reaction. Indeed, the cells are increased in numerous pathologic conditions, including infections, inflammatory diseases, graft-versus-host disease, traumatic stress, and neoplastic diseases (Dolcetti et al., Cancer Lett. 2008 Aug. 28; 267(2):216-25; Talmadge, Clin Cancer Res. 2007 Sep. 15; 13(18 Pt 1):5243-8).